On large contracts the concrete is generally mixed by machinery. The economy is not only in the mixing itself but in the appliances introduced in handling the raw materials and the mixed concrete. If all materials are delivered to the mixer in wheel-barrows, and if the concrete is conveyed away in wheel-barrows, the cost of making concrete is high, even if machine mixers are used. If the materials are fed from bins by gravity into the mixer, and if the concrete is dumped from the mixer into cars and hauled away, the cost of making the concrete should be very low. On small jobs the cost of maintaining and operating the mixer will usually exceed the saving in hand labor and will render the expense with the machine greater than without it.

136. Machine Vs. Hand Mixing

It has already been stated that good concrete may be produced by either machine or hand mixing, if it is thoroughly mixed.

Tests made by the U. S. Government engineers at Duluth, Minn., to determine the relative strength of concrete mixed by hand and mixed by machine (a cube mixer), showed that at 7 days, hand-mixed concrete possessed only 53 per cent of the strength of the machine-mixed concrete; at 28 days, 77 per cent; at 6 months, 84 per cent; and at one year, 88 per cent. Details of these tests are given in Table X.

It should be noted in this connection, that the variations in strength from highest to lowest were greatest in the hand-mixed samples, and that the strength was more uniform in the machine. mixed.

Table X. Tensile Tests Of Concrete

(From "Concrete and Reinforced Concrete Construction," by H. A. Reid)

The mixture tested was composed of 1 part cement and 10.18 parts aggregate.